The immediate early protein tristetraprolin (TTP) is induced by numerous stimuli and during some apoptotic events, and is part of a zinc finger protein family that is generally involved in RNA binding. In mice, lack of TTP causes a systemic inflammatory and myeloproliferative syndrome that is mediated by TNFalpha. Some studies suggest that TTP may bind and destabilize the TNFalpha mRNA directly, but other evidence indicates that TTP influences different pathways that could affect TNFalpha regulation or responses. We have determined that continuous expression of TTP at approximately physiological levels triggers an apoptotic response, apparently analogously to some oncogenes. The closely related TIS11b and TIS11d proteins have similar effects, but low-level TTP expression in particular sensitizes cells to TNFalpha-induced apoptosis, suggesting that lack of TTP might lead to an overexuberant response to TNFalpha in vivo. The data suggest that TTP influences critical regulatory pathways when it is induced during growth factor responses, and that the effects of TTP on TNFalpha regulation may be complex. The apoptotic effects of TTP are enhanced by specific binding of 14-3-3 proteins to phosphorylated TTP, suggesting that 14-3-3 binding influences TTP localization or stability, or promotes TTP activity directly. We propose the following series of complementary experiments to investigate how TTP functions at the molecular level. We will conduct structure/function analyses of how TTP triggers apoptosis, is localized within the cell, and sensitizes cells to TNFalpha-induced apoptosis. We will also investigate how TTP influences regulatory pathways that are suggested as candidates based upon its effects on the TNFalpha response, and on parallels between TTP and certain oncogenes. We will investigate how 14-3-3 isoforms recognize TTP and promote its apoptotic activity, and we will continue a methodical two-hybrid and biochemical approach to identify additional proteins that interact with TTP and contribute to aspects of its function, particularly its binding to RNA.